Thioesters and process



Patented Feb. 9, 1954 THIOESTERS AND PROCESS Lawrence W. Newton, South Charleston, W. Va.,

assignor, by mesne assignments, to Union Car- "bide and Carbon Corporatiomacorporation-of New York No Drawing. ApplicationJanuaryZB, 1948, Serial No. .4,9. 52

.12 Claims. (Cl. 260-481) This invention :relates to the production or a new class of esters of thiopolycarboxylic acids; and more especially it concerns the production of polyesters of .thiotetracarboxylic acids and thiohexacarboxylic acids, many of which have utility as ,plasticizers for vinyl resins and for other synthetic resinous plastic materials such as cellulose acetate andother cellulose esters of the lower fatty acids. The invention also relates to a new class'of thiopolycarboxylic acids which can be formed by the hydrolysis of the corresponding esters.

.Esters of the new class are produced, in accordance with the invention, by reacting a diester of ,an mfi-unsaturated diearboxylic acid, and especially those having conjugated double bonds, or a corresponding triester of an afiunsaturatedolefinic tricarboxylicacid, with hydrogen sulfide at a temperature within the range between C. and 300 C. When employing a basic condensation catalyst in the process it is preferred to conduct the reaction at temperaturesbetween 0 :C. and 100 C. In the'case of an .uncatalyzed reaction, higher temperatures Within the rangefromaround 150 C. to300 C. preferably are used with pressures above atmospheric. The polyesters oi the resultant saturated-thiopo1ycarboxy1ic acid are recovered from the resultant reaction mixture, preferably by fractional distillation under high .vacuum.

The new compounds are saturated aliphatic monothiodipolycarboxylic acids having at least 8 carbon atoms, and containing only carbon, hydrogen, oxygen and sulfur in the molecule, and the .tetraand hexaesters .of such acids wherein the hydrogen atom of each carboxyll groupis replaced by a radical selected from the class consisting of the alkyl, aralkyl, alkoxyalkyl, aryloxyalkyl, cycloalkyl, halogenated alkyl, alkoxyalkoxyalkyl, aryloxyalttoxyalkyl and alkyL ene radicals.

The preferred estersof the-newclass of compounds have structures apparently correspondingtostheformula "olrooon .Homooom wherein :R' maybe either hydrogen, a lower alky1 group orua ---.-CI-Mallryl') COO'R group; B may Joe aeither :hydrogen, ualkyl, a -GH2'COOR group or a -CI-I(alkyD'GOOR group; R ebeing hydrogenor'alkyl when R is either -CH2G00R or CH(all yl)COOR and E being hydrogen or alhylwhen R" is--CH(al-kyl0 0003. and R' R R and R are the same or different alkyl, aralkyl, cycloalkyl, alkoxyalkyl, ary'loxyalkyl, halogenated al-kyl, alkoxyal-koxyalhyl, -aryloxyalkoxyalkyl, or alkylenegroups.

The reaction with hydrogen sulfide may :he conducted in the presence of a solvent for rthe reactants which is inert to the latter. The use of such a solvent is GesirabIe- 'Whentheesterof the olenfinic polycarboxylic acid is a solid under the reaction conditions, or :when .such ester a poor solvent for hydrogen sulfide, or :for lithe catalyst, whenemployed. fiuitablersolvents found useful :a-re aliphatic alcohols, such as ethanol and loutanol; others, such as .diethylxether, the

mono-- and tdialkyl ethers of the glycols sand 1 polyglycols, and vdiOXfiIlG; and :aromatic 1113711110- carhons such as benzene.

Although the reaction will proceed .rslowly in the ,absenceof a catalyst, it :usually desirable to conduct the reaction in attic presence :uf a. catalyst :in order ;to cause :the reaction :to 111110- .ceedzat a commercially practicable rateinsimple apparatus at atmospheric pressure. The horn catalytic reaction usually requires the [use of high temperatures and superatmosphericpressure in pressure vessels. .If desired, ,;p1'-essures above atmospheric may be used in s the presence of a. catalyst.

Catalysts :suitable vfor use in the process :are relatively strongly :basic compounds. Those, such ,as ammonia, sodium hydroxide, etc. which react with theester of :the .olefinic, polycarboxylic acid .to form weakhaseslare not as ,efiectiveas the preferred catalysts. The latter include the strong organic bases, such .as piperidine, the secondary an'dtertiary aliphaticamines such as diethylamine and .triethylamine; and the quaternary ammonium "bases suchas trimethylbenzxil ammonium hydroxide. Weakorganic'bases such as pyri'dene are less effective as catalysts. "Inorganic bases and 'inorganicsor anic "basessuQh as sodium hydride and sodiurnbutylate are effective but are difiicultlycremovable from the reaction products. Ammonia .and the primary amines such as butylamine show relatively poor catalyst activity, probably due=to-reaction=thereor with the ester starting material to form-weak bases. Neutralizefd "bases such as ammonium carbonatedo not function as catalysts.

The reactions involved in -the production ot 7 reaction tube fitted with a glass diffuser.

the polyesters of certain saturated thiopolycarboxylic acids are represented by the followin equations:

cn oooar HoR oooR His CRCOOR4 HS-CR'COOR4 CR'COOR ncmcoon: HomcooR RCOOR HSCRCOOR CRCOOR4 a ive 0R5 HC R20 0 0R6 wherein R to R designate the radicals previously mentioned.

As shown, the reaction apparently occurs in two stages, the first forming a mercapto derivative of the saturated dicarboxylic acid corresponding to the unsaturated dicarboxylic ester starting material. The second reaction forming the ester of the saturated thiodi(polycarboxylic)acid usually is much faster than the first reaction when catalyzed by the aforesaid catalysts so that the final product consists essentially of the tetraor hexaester of the thiodipolycarboxylic acid containing only a small amount of the mercapto compounds.

In a preferred practice of the invention successive portions of hydrogen sulfide are difiused through a. mixture of the diester of the selected r p-unsaturated olefinic polycarboxylic acid and the catalyst while maintaining the reaction temperature between 0 C. and 100 C. When no catalyst is used, temperatures as high as 150 C.

or higher conveniently can be used in conjunction with superatmospheric pressures, and the yields of the desired ester are relatively low. The ester and catalyst may be dissolved in an inert volatile solvent when desired. The resultant reaction mixture then is fractionally distilled under vacuum to remove solvent and other volatiles. The fraction containing the desired ester is distilled under high vacuum, usually in a falling film type of molecular still, yielding a purified polyester of a saturated thiodi(polycarboxylic)acid. Among the polyesters of cup-unsaturated ole- 'finic diand tricarboxylic acids useful in the process may be mentioned the dialkyl, cliaralkyl, dialkoxyalkyl, diaryloxyalkyl, dihalogenated alkyl, dialkoxyalkoxyalkyl, diaryloxyalkoxyalkyl, dicycloalkyl and dialkylene esters, such as dimethyl, diethyl, di-isopropyl, dibutyl, di-n-hexyl, -di-2-ethylhexyl, di--ethylnonanyl-2, dicyclohexyl, dibenzyl, di-methoxyethyl, di-phenoxyethyl, di methoxyethoxyethyl, di phenoxyethoxyethyl, di-p-chloroethyl, di-p-bromoethyl, divinyl and diallyl esters of such acids as maleic and fumaric acids; methylene malonic and ethylidene malonic acids; alkylmaleic acids, such as methylmaleic (citraconic) acid and ethylmaleic acid; itaconic acid; mesaconic acid; and the corresponding triesters of a,B-unsaturated olefinic tricarboxylic acids such as aconitic'acid, and the aand 'y-alkyl aconitic acids.

i In the following examples which illustrate the invention, all parts recited are in terms of weight unless otherwise indicated.

EXAMPLE 1 Tetramethyl thiodisuccinate During 4.5 hours hydrogen sulfide was diffused -;through a mixture of 1100 parts of dimethyl maleate and 33 parts of triethylamine in a glass During this...period 112 parts of hydrogen sulfide, were absorbed, and the reaction temperature was held at 70 C.- C. The liquid reaction products were stripped of volatiles under vacuum and the residue was distilled in a falling film type molecular still. The fraction distilling at 175 C. under a pressure of 0.5 millimeter of mercury was tetramethyl thiodisuccinate in the form of a viscous liquid having a refractive index at 20 C. of 1.4779 and a specific gravity at 30 C. of 1.255. Upon standing several weeks the ester crystallized. Recrystallization thereof yielded a white solid melting at 42 C.

EXAMPLE 2 Tetrabutyl thiodisuccinafe During 4 hours 33 parts of hydrogen sulfide were absorbed in a mixture of 417 parts of dibutyl maleate and 5 parts of triethylamine contained in a glass reactor tube provided with a hydrogen sulfide diffuser while maintaining the mixture at temperatures ranging between 43 C. and 57 C. The reaction products were stripped of volatiles in a vacuum still to a kettle temperature of C.- C. under 1 millimeter of mercury pressure. The residue was distilled in a molecular type still. The fraction distilling at 156 C. under 0.05 millimeter of mercury pressure was tetrabutyl thiodisuccinate. It had the other properties recited in Table I.

EXAMPLE 3 Tetrabutyl thiodisuccinate A glass reactor provided with a cooling coil was charged with 1200 parts of dibutyl fumarate, 600 parts of butanol and 18 parts of triethylamine, and an excess of hydrogen sulfide over that required to produce tetrabutyl thiodisuccinate was passed through this mixture for six hours. After standing for about 60 hours the reaction mixture was stripped of butanol and other volatiles in a still to a kettle temperature of 186 C. under 2.4 millimeters of mercury pressure. A yield of 1255 parts of crude tetrabutyl thiodisuccinate was secured, corresponding to a yield of around 97% based upon the dibutyl fumarate charged.

EXAMPLE 4:

912 parts (4 mols) of dibutyl maleate and 150 parts (4.4 mols) of hydrogen sulfide were charged in a steel pressure vessel. The latter were sealed and the mixture was heated to 150 C. for 15 hours with agitation, under self-induced superatmospheric pressure. The reaction mixture then was fractionally distilled in a Claisen type still to a temperature of C. under a pressure of 3.7 millimeters of mercury. The still residue. crude tetrabutyl thiodisuccinate. was redistilled in a falling film type molecular still, yielding purified tetrabutyl thiodisuccinate.

EXAMPLE 5 Tetra-n-hezryl thiodisuccinate A stream of hydrogen sulfide was diffused during twelve hours through a mixture of 970 parts of di-n-hexyl maleate and 11 parts of triethylamine held at 40 C.60 C. During the reaction an extra 14 parts of triethylamine were added. The resultant reaction mixture was stripped of volatiles in a Claisen type still to a kettle temperature of 256 C. at 3.6 millimeters of mercury pressure. A yield of 920 parts of crude tetrahexyl thiodisuccinate was secured which, upon redistillation in a molecular type still, boiled at aeemsav 183 C. under 25 microns er-mercury :Lpressure, andhadthe other propertiescrecited in" Table :I.

Tetra(2-ethylhemyl) thiodisuccinate During 5 hours 21 parts of hydrogen sulfide were absorbed i 378 parts of, .di(2ethylhexyl) maleate containing '1 parts of triethylamine, maintained at 39 C.-53 G. The-resultant reaction mixture was stripped in a =Claisen typestill to 200 C. kettle temperature under 2.5 millimeters of mercury pressure, leaving as residue 376 parts of tetra(Z-ethylhexyl) thiodisuccinate. Upon distillation of "the latter in a falling film type molecular still a purified -tetra(2-ethylhexy1)thiodisuccinate was obtained having the properties recited in Table I.

EXAMPLE Tetra(fi-ethylnonanyl-z)thiodisuccinate Following the general procedure of Example 6, hydrogen sulfide was difiused during hours within a mixture of 402 parts-of 'di(-5-ethylnonanyl-2)-maleate and 7 parts of triethylamineheld at about 100 -C. The reaction mixture was stripped of volatiles .in a Claisen type still to a kettle temperature of 223 C. under a .pressure of 2 millimeters of mercury, and the still residue was-iractionally distilled under high vacuum in a-falling film-type-molecularstill. The resultant purified tetra(5-ethylnonanyl-2)thiodisuccinate obtained was an-oily liquidihaving the properties recited in Tabla-I.

Tetra ()3methomyethylithiodisuccinate During 2 hours hydrogen sulfide was diffused through a mixture (if-464 parts of diqa-methoxyethyl)maleate and 10.6 {parts of -triethylamine in a glass reaction tube, 41lparts of the hydrogen sulfide being absorbed. The temperature ranged between C. and 63 C. The reaction mixture was stripped in a pot still to a kettle temperatureof 145 C. at-one millimeter of mercury pressure, yielding LBS-parts of crude tetra-methoxyethyD'thiodi'Succinate. Upon"fractionally distilling the crude material in a molecular still, the purified ester was secured as a viscouspartially Water-soluble liquid, distilling at 220" (Let 0.2 millimeter of mercury pressure, and having the other properties Jrecitedin Table I. 'The ester crystallized on standing, and the recrystallized material melted at 42 C.

EXAMPLES) Tetra(,9-pheno:z:yethyl)thiodisuccinate "During ll;5 hourshydrogensulfide was diffused througha "mixture "of 362 "parts of elitephenoxyethyl) maleate, 400 partsof benzene, and 318 parts of tri'ethylamine' within a'glas's reaction tube maintained at 50" CA-75 C. The reaction mixture was stripped o'f' volatiles by distillationto akettle temperature of 150 C. at' 's millimeters of mercury pressure, yielding 7 7 parts of "crude tetra(fl-plienoxyethyl)thio'disuccinate in the formof a liquidto'o v-iscous at room temperature to pour. The "material "eventually crystallized to a white selidwhi'ch,- after recrystallization, melted at 65 C.-6'7 C.

Tetrale-ehloroethiil)thjiodisuccindte During "3 hours -hy'drogen 'sulrl'de We 'di'fiu'sed within 'a mixture :01, 241 parts of dles-chlamethylimal'eate :dissolved :in 440 parts of benzene and containing 4 parts 10f triethylamine :maintained at a temperature ranging between 30 6. and 45 C., 24 parts of hydrogen sulfide being absorbed. The benzene was stripped from the reaction mixture is 'a still to a kettle temperature of C. at 2 millimeters of mercury pressure, yieldin'g259 parts of crude tetra(p-chloroethyl) thiodisuccinate. Upon recrystallization of the crude product from methanol the puri'fie'd ester was secured as a white crystalline soli'd melting at 84 C.

Tetra(2-ethylbutylythiodisuccinate During 4 hours hydrogen sulfide was dlfiused through a mixture of 606 parts of di(2-ethy'lbutyDmaleate and 6'parts ofpiperidine in'a glas's reaction tube maintained at 51 C.-'-66 CC "450 parts of hydrogen sulfide being absorbed. The crude reaction mixture was stripped by distillation to a kettle temperature of 147 C. at a pressure of 0.5millimeter of mercury; and the-residue was distilled in 'a molecular type still, yielding purified tetra(2-ethylbutyl)thiodisuccinate in the form of a light yellow liquid having the, properties recited in TableI.

EXAMPLE12 Tetraallyl thz'odisu'cci'nate Hydrogen sulfide was diffused during onehour through a mixture of 2 mole of diallyl .maleate containing 1% of diethylamine catalyst and 0.l'% of lead acetate, maintained at betweenBO" 0.:and 20 C., slightly more than one mol of hydrogen sulfide being absorbed. The lead acetate served as a polymerization inhibitor. Thecrudereaction mixture was iractionally distilled in a falling film type molecular still, providing an 825% yield of refined tetraallyl thiodisuccinate in the form of an oily straw-colored liquid boiling at 149 C. at 0.4 millimeter of mercury pressure, and having "the other properties recited Table '1.

EXAMPLE l3 Thz'odisuccinic acid .A mixture of 0.5 mol of tetramethyl :thiodisum cinate, 200 cc. of water and ice. of concentrated sulfuric acid was refluxed in a .stripping stilliuntil methanol ceased to distil over. Water was added periodically to maintain a constant level in the still kettle, while-aqueous methanol was removed overhead. The hydrolysis required -8 hours. The residual product was recrystallized three times from water to give a 62% yield 0! white crystalline thiodisuccinic .acid having a melting point of 212 C.-2l3 C., a purity of 99.7% by titration, and -'a solubility in water of "816% at 20C.

The tetraethyl and higher esters of this acidare increasingly resistant to hydrolysis.

EXAMPLE 14 'Heaaethyl thio'di(tricafbdllzilate') l-ly'dro'geri sulfide was 'lpassedthrough a glass reaction"tubelfittedfwith a glass diffuser andrcontaining 750 parts of triethyl aconitate :and "IE5 parts ofdiethylamine, maintained between 27 6.- 59 C. During 4.5 hours 'op'erationSO .parts .of hydrogen sulfide were reacted. The iicrude reaction mixture was idistilledin 'a falling film type molecular still, yielding a refinedhexaethyl thl- 8 This material was compatible with a vinyl chloride-acetate copolymer resin at 33% concentration to give a clear but rather stifi sheet.

odi(tricarballylate) in the form of an oily liquid boiling at 171 C. under 3 microns of mercury pressure, and having the other properties recited in Table I. Table II illustrates the relative effectiveness TABLE I Refractive Index S ocifio Gravit Compound Boiling Point, C. T p T y 'Ietramethyl thiodisuccinate 42 175 at 0.5 mm. pressure 1.4779 at 20 C 1.2553 at 30 C. Tetraethyl thiodisuccinate 222 at 5.5 mm. pressur 1.4643 at 20 C 1.1491 at 20 C. Tetrabutyl thiodisuccinate... 156 at 0.05 mm. pressure.. 1.4589 at 30 1.0545 at 20 C. Tetrahexyl thiodisuccinate 183 at 25 microns pres 1.4596 at 30 C... 1 0083 at 20 C.

09821 at 20 C.

Tetra (Z-ethylhexyl) thiodisuccluate 0.955 at 20 C.

183 at 17 microns pres 207 at 20 microns pre Tetra (,B-methoxyethyl) thiodisuccinate 42 220 at 0.2 mm pressure... 'IetraQi-phenoxyethyl) thiodisuccinate 05-67 Thiodisuccinic acid Tetraisopropyl thiodisuccinate 185 at 1.5 mm. pressure. 1.072 at 20 C. Tetra(2-ethylbutyl) thiodisuccinate 231 at mm. pressure 1.016 at C. Hexaethyl thioditricarbally1ate. 171 at 3 m crons pres 1.160 at C. Tetra(2, 6-dimethylheptyl-4) thiodi 156 at 2 microns pres 0.942 at 20 C. Tetrabutyl thiodi(a-methylsuccinate). 225 at 1 5 mm pressure. 1.046 at 20 C. 'letraallyl thiodisuceinate 149 at 0.4 mm. pressure 1.149 at 20 C. Hexabutyl tl1iodi(tricarballylate) 183 at 5 microns pres 1.4645 at 20.5 0..." 1.0622 at 20 C.

In similar manner hydrogen sulfide was diffused through tributyl aconitate containing 1% diethylamine and about 25% of benzene during 3 hours at 32 C.-5l C. The reaction mixture was stripped in a pot still to a temperature of 175 C. at 2 millimeters of mercury pressure and the residue distilled in a molecular type still, yielding hexabutyl thiodi(tricarballylate) as an oily liquid boiling at 183 C. at 5 microns of mercury pressure, and having the other properties recited in Table I. It was compatible with a vinyl chloride-vinyl acetate copolymer resin to give a clear flexible sheet containing 33% of the compound.

EXAMPLE 15 Tetrabutyl thz'odi(a-methylsuccinatel During 15.5 hours hydrogen sulfide was passed through a mixture of 600 parts of dibutyl itaconate and 6 parts of triethylamine in a glass reaction tube maintained at C. An additiona1 6 parts of triethylamine and 6 parts of diethylamine were added during the reaction period; and 21 parts of hydrogen sulfide were absorbed. The crude reaction mixture was stripped of volatiles up to a kettle temperature of 150 C. atone millimeter of mercury pressure; and the residue therefrom was distilled in a falling film type molecular still to provide 171 parts of refined tetrabutyl thiodi(a-methylsuccinate) as a light yellow liquid boiling at 225 C. at 1.5 millimeter of mercury pressure, and having the other properties recited in Table I.

EXAMPLE 16 Tetrabenzyl thiodisuccinate Hydrogen sulfide was diffused through a mixture of 195 parts of dibenzyl maleate, 400 parts of benzene, and 5 parts of diethylamine. During the first 10 minutes the temperature rose from 25 C. to 50 C. and then subsided gradually during the remaining portion of the 1.5 hour reaction time. The reactor and its contents absorbed 29 parts of hydrogen sulfide as judged by gain in weight. The benzene solvent was stripped oil in a Claisen type still to a kettle temperature of 205 C. at 2.5 millimeters of mercury, yielding 204 parts of a yellow syrupy liquid which crystallized on standing. A portion of this product was recrystallized from ethanol, yielding the white waxy solid tetrabenzyl thiodisuccinate with a freezing point of 49.2" C.49.4 C.

of various catalysts of the type herein described in the production of tetrabutyl thiodisuccinate from dibutyl maleate and hydrogen sulfide in accordance with the present invention. In the runs there recorded hydrogen sulfide was diffused through a 1% solution of the catalyst in 500 grams of dibutyl maleate contained in a small unpacked Pyrex column. The activity of each catalyst was determined by observing the rate at which hydrogen sulfide was absorbed, the maximum temperature elevation, the length of time required to complete the reaction, and the yield of the tetra-ester. In certain runs butanol was used as a solvent.

TABLE II Preparation of tetrabutyl thiodisuccinate EFFECT OF CATALYSTS Peak Comple- Tetraggg g Tetmltion 3lflstleir pera ure, ime, ie Grams C. Hrs. Percent 'lrimethylbenzyl ammonium hydroxide 35 100 1. 75 96.0 32 82 3. 0 96. 6 35 87 3. 0 96. 2 30 98 3. 0 96.0 3. 0 98. 6 Diisopropylamine 31 70 40 95. 5 Ammonia 27 63 93. 4 Sodium hydride 25 51 3.4 60.6 Sodium butylate 8 81 3. 0 28. 8

The tetraalkyl esters of thiodisuccinic acid are valuable plasticizers for vinyl resins such as vinyl chlorides, copolymers of vinyl chloride and vinyl acetate, and copolymers of vinyl chloride and vinylidene chloride. Upon milling a mixture of 33 parts of each of the tetraethyl, tetrabutyl and tetrahexyl esters of thiodisuccinic acid with 2 parts of a well-known light stabilizer such as dibutyl tin dilaurate and 65 parts of a vinyl chloride-vinyl acetate copolymer resin containing around 96% of the chloride in the polymer, using a small two-roll mill, clear flexible sheets were obtained which showed no evidence of sweating out of the plasticizer on standing for several months.

Table III lists some properties of certain molded specimens of mixtures of the aforesaid vinyl resin and various tetraalkyl thiodisuccinates of the invention. The compatibility of the resin with the plasticizer was 100% in each case.

9 fit TABLE III.

Plasticizer evaluation CHARACTERISTICS OF MOLDED RESIN SPECIMENS Plwi. Percent Extraction H ASTM Tensile Elonga- Ncnie gifig TF, C. V Stiff. Strength, ticn,

Percent at Mod. [1.5. 1. Percent Tetraethyl thiodisuccinate 33.0 1 l.7. i 12. o 1229 750' 2, 7150 s35 'Ietraisopropyl thiodisuccin 40.8 -13. 5 8. (l 2 52 5 2, 530 330 Tetrabutyl thiodisuccinate 38. 0 -2713 l7. 0 0. 34 575 2, 52Q 332 Tetra(2-ethyl-butyl)thiodisuccinate. 40. 5 20. 5 15. 0 0.5 740 2425 350 Tetrehexyl thiodisuccinate; as. s 32. 2s. 0 i 0. 40.. 700' 2, 350 330 The tetramethyl ester of thiodisuccinic acid is compatible with the vinyl. resin, but sweat-out difiiculties arise. The tetra.(2-ethylhexyl) ester of thiodisuccinic acid is compatible with the resin in 33% plasticizer concentration, but this is near the borderline of compatibility therewith. The tetrabutyl thiodisuccinate appears to be a most valuable plasticizer of this series of compounds for use with the polyvinyl chloride resins and the vinyl chloride-vinyl acetate copolymer resins. This particular compound possesses properties very similar to dioctyl phthalate in most respects affecting its plasticizing efiiciency for the resins, while being superior to the latter in evaporation rate, heat stability, and milling life. The hexaethyl and hexabutyl esters of thioditricarballylic acid also are compatible with copolymers of vinyl chloride and vinyl acetate in up to at least 33% of the resinous composition. The lower tetraalkyl and tetraalkoxy alkyl thiodisuccinates such as tetramethyl and tetra- (methoxyethyl)thiodisuccinates are compatible with cellulose acetate, and impart to the latter properties generally similar to those imparted thereto by diethyl phthalate, which is a widely used commercial plasticizer for cellulose acetate.

The following illustrates the plasticizing efficiency of tetrabutyl thiodisuccinate for vinyl resins:

A plastic composition containing 67% of a vinyl resin, 1% of lead stearate, 2% of litharge, and 30% of tetrabutyl thiodisuccinate was prepared on a two-roll mill, and was examined to evaluate the last-named compound as plasticizer. The resin was a copolymer of vinyl chloride and vinyl acetate containing 96% of the chloride in the polymer. The composition had a tensile strength of 3400 p. s. i., an ultimate elongation of 250% under a stress of 15,000 p. s. i./ minute, and an elongation of 45% under a stress of 1,000 p. s. i. A film 0.004 inch thick made from the composition was unaffected after im mersing in water at 25 C. for days; and it lost 4.2% of its weight during immersion in oil at 25 C. for the same period. It lost 0.4% of its weight upon exposure for 10 days in air at 60 C. This plasticizer is non-toxic, and has a plasticizing efiiciency equalling that of dioctyl phthalate.

The invention is susceptible of modification within the scope of the appended claims.

I claim:

1. An ester or a thiopolycarboxylic acid having a structure represented by the formula ncmooom oR'coon s geon'cooeu homoooru;

wherein R is: a radical selected from the class consisting. of hydrogen, the lower alkyl groups and the '--CH2COOR grdups; R is a radical 'ieleeted from the class consisting of hydrogen; the lower alkyl groups and the CH2COOR3 groups, but wherein R and R do not concurrently designate a radical having a --COOR. component; and each R is a radical selected from the class consisting of the alkyl, aralkyl, alkoxyallryl, aryloxyalkyl, halogenated alkyl, cycloalkyl, alkoxyethoxyethyl, aryloxyethoxyethyl and alkenyl radicals.

2. As new compounds, esters of saturated aliphatic symmetrical monothiodipolycarboxylic acids, the total acid radicals of which together have from eight to twelve carbon atoms and contain only carbon, hydrogen and one sulfur atom in addition to the carboxyl carbon and oxygen atoms, said compounds having from four to six alkyl-substituted carboxyl groups respectively attached to dififerent carbon atoms.

3. As new compounds, the tetraalkyl esters of symmetrical monothiodisuccinic acid, each of the alkyl groups containing from one to eleven carbon atoms.

4. As new compounds, the tetraallroxyethyl esters of symmetrical monothiodisuccinic acid.

5. The tetraalkyl esters of thiodiialkylsuccinic) acids wherein the said alkyl radical of the alkylsuccinic group has not more than three carbon atoms.

6. Tetrabutyl thiodisuccinate.

'7. The hexaalkyl esters of a saturated aliphatic symmetrical monothiohexacarboxylic acid wherein the six esterified carboxyl groups are attached respectively to different carbon atoms.

8. The hexaalkyl esters of thiodi(tricarballylic) acid.

9. Process for producing polyesters of thio-- polycarboxylic acids, which comprises reacting hydrogen sulfide at an elevated temperature with a polyester of an rip-unsaturated olefinic polycarboxylic acid, which polyester contains from two to three esterifled carboxyl groups in the molecule, and in the absence of added water, and recovering from the resultant reaction mixture the polyester of a symmetrical monothiodipolycarboxylic acid thus produced.

10. Process for producing polyesters of thiopolycarboxylic acids, which comprises reacting polycarboxylic acids, which comprises reacting and condensing hydrogen sulfide with a polyester of an aliphatic ark-unsaturated polycarboxylic acid, which polyester contains from two to three esterified'carboxyl groups in the molecule, in the presence of an alkaline nitrogen-containing compound selected from the class consisting of the primary, secondary and tertiary amines, ammonia and the quaternary ammonium bases, and in the absence of added water, and recovering from the resultant reaction mixture the polyester of a thiopolycarboxylic acid thus produced.

12. Process for producing polyesters of thiopolycarboxylic acids, which comprises reacting and condensing hydrogen sulfide with a diester of an rip-unsaturated olefinic dicarboxylic acid which acid has between four and six carbon atoms in the molecule, in the presence of an organic base as catalyst and in the absence of added water, and recovering from the resultant reaction mixture the polyester of a symmetrical 1 monothiodipolycarboxylic acid thus produced.

LAWRENCE W. NEWTON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,652,099 Douglass Dec. 6, 1927 2,121,617 Werntz June 21, 1938 2,176,423 Jaeger Oct. 17, 1939 2,212,141 Alderman Aug. 20, 1940 2,268,185 Burke et al. Dec. 30, 1941 2,356,586 Hentrich Aug. 22, 1944 FOREIGN PATENTS Number Country Date 845,793 France May 22, 1939 

1. AN ESTER OF A THIOPOLYCARBOXYLIC ACID HAVING A STRUCTURE REPRESENTED BY THE FORMULA 